In recent years, electronic devices that can be operated by touching a screen with a finger, a pen, etc., have proliferated. In addition, for a method of detecting a touch position on a display screen, a method is known in which a plurality of optical sensors are provided in a display panel and a shadow image which is created when a finger or the like approaches a screen is detected using the optical sensors.
In relation to this, Patent Document 1 describes a display device in which a backlight is turned on during a display period for displaying display data on a display unit and the backlight is turned off during a sensing period during which sensor outputs from sensor units are read. According to the display device, the influence of backlight light upon detecting a shadow image is reduced, enabling to improve the detection accuracy of optical sensors.
However, in the method of detecting a shadow image, when the illumination of outside light is low (the surroundings are dark), it may become difficult to distinguish between a shadow image and a background in an image obtained by the optical sensors and accordingly a touch position may not be able to be detected properly. Hence, for display devices including a backlight, a method is also devised in which a reflection image which is created when backlight light hits a finger is detected using optical sensors.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2006-317682